The rabbit cortical collecting tubule is thought to play an important role in the regulation of Na, Cl and K excretion. Isolated perfused tubules of this nephron segment absorb Na and Cl from the tubule lumen and secrete K into the lumen. The aim of the present proposal is to elucidate the cellular mechanisms which are responsible for transport of these ions in the isolated tubules. Conventional and ion-selective microelectrode will be employed to impale individual cells to directly measure intracellular voltages and ionic activities and hence, the electrochemical gradients for each ion. These estimates will be combined with measurements of transepithelial voltages, conductances and isotopic ionic fluxes to evaluate the distribution of fluxes between transcellular and paracellular pathways, the passive ionic permeabilities and associated ionic fluxes across individual cell membranes, the active transport steps, and the coupling of flows between ions. In addition, coupling of Na and K fluxes across the basolateral membrane and the relation of these fluxes to the Na-K-ATPase activity (the enzymatic equivalent of the Na:K exchange pump), will be assessed to establish the relation of the Na:K exchange pump to the transepithelial movement of these two ions. The studies should provide important new insights into the intraepithelial location and cellular mechanisms of ion transport by the rabbit cortical collecting tubule.